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Full text of "Handbook Of Chemical Engineering - I"

276
CHEMICAL ENGINEERING
the uppermost machine after overflowing would pass to the machine next below and be used for wash water for this second machine, etc., thus employing the principle of Ostwald of the least use of wash water.
CAPACITY OF DOBR AND AKINS CLASSIFIEES PER FOOT OF WiDTH2
Mesh1, of separation desired	Overflow capacity, tons dry per 24 hr.	Raising capacity, tons dry per 24 hr.	Dorr, strokes per minute	Akins, revolutions of spiral per minute	Dorr slope in. per ft.	Akins slope		Water to pulp in feed
28	65 to 85	130	25 to 30	15 to 20	3Kto3M	& CO		2:1
48	60 to 70	125	25 to 30	15 to 20	3    to 3M			3:1
65	40 to 50	100	20 to 25	12 to 15	2% to 3			3.5:1
100	30 to 40	90	16 to 20	10 to 12	2H to 2%			4-6:1
150	20 to 30	75	12	8	2    to2J4			6-10:1
200	10 to 20	60	12	6	2    to 2^J-	& <M		10-15:1
Separation by Settlement and Decantation or Overflow.—The chief advance in these devices has been in mechanical means for aiding in the discharge of the settled material. Long V-shaped tanks have been provided with valves which open intermittently, the mechanism being a pulley the face of which is wanting at one or more portions of the circle. During the revolution of the faced portion of the circle of the pulley the spring valve is held by it against the opening of the tank. When the recessed portion of the pulley rotates to a point facing an opening the spring pressure is removed, the valve opens and the contents of the tank discharge. With these tanks the material fed passes in at one end and overflows at the other. The material which settles is graded from coarse at the entry end of the feed to the very finest which is capable of settling at the discharge end. The intermittent device works poorly on 12-mesh material and begins to be uncertain in action with 20-mesh material owing to chokage unless oversize plugs are used when the escape of water when the valves are opened is so great that very little effect is secured from discharging intermittently. Plug openings range in size from % in. to % in.
Continuous Cone.—As shown in Fig. 8 the stream of material enters the feed .spout A and, after passing through the truncated cone B, the water and solid particles which it is desired to remove flow upward and out into the overflow launder. The heavy particles settle in the cone and form the basin. When the settled material reaches the outlet of B it obstructs it so that the water rises in B lifting the float C, thereby lowering the ball G from the spigot by means of the lever D, the link E and the valve arm F. Once the cone is filled up a balance will be obtained and the settled solids will flow out in a continuous stream but the
1 This should be the average of the first two meshes which report in the percentage by weight column which gives a greater average than 2 per cent, the separation mesh being then assigned to the screen of most openings of the pair involved in the averaging. In the case of the Timber Butte figures the separation is at 65 mesh, and of the United Eastern and Utah Copper mills at 28 and 35 mesh respectively. 2 The widths of the Akins machine are the diameters of their spirals or dimensions M in the table